This invention relates to the electrolytic polishing of the inside surfaces of hollow niobium bodies in general, and more particularly to an improved method of carrying out such polishing.
As disclosed in U.S. patent application Ser. No. 289,543 filed Sep. 15, 1972 and assigned to the same assignee as the present invention, the inside body of a hollow niobium body having at least one opening, can be polished by immersing the niobium body partially in an electrolyte containing H.sub.2 SO.sub.4, HF and H.sub.2 O. The niobium body acting as an anode is arranged rotatably about an axis of rotation extending through the opening such that for any position of the hollow niobium body, a coherent empty space exists, which is in communication with the outside environment through the opening of the body. Thus, all portions of the body between the electrolyte level and the inside of the hollow body are in communication with the outside environment. To carry out polishing, individual parts of the inside surface are successively immersed in the electrolyte but with no part of the inside surface remaining continuously therein. Electrolytic polishing is accomplished through the use of a cathode introduced through the opening into the hollow niobium body and arranged in the electrolyte in such a manner that the region of the electrolyte in which gases are formed at the cathode during current flow, rise to the surface of the electrolyte to an area free of parts of the inside surface of the hollow niobium body so that they may escape the outside environment. In addition, in carrying out this polishing, a constant electric voltage is applied across the hollow niobium body acting as an anode and the cathode with the electric voltage being such that damped current oscillations are superimposed on the electrolyte current. In addition, the voltage is switched off no later than at the point of complete decay of the current oscillations to permit the oxide layer which was built up during the oscillations to be dissolved. Thereafter, the constant voltages again applied resulting in damp current oscillations, another step of dissolving performed and so on, with the steps repeated several times. In each case, the hollow niobium body is kept at rest during dissolution of the oxide layer and is then rotated about the axis of rotation before voltage is again applied.
In basic terms, this method of electrolytic polishing of niobium parts is disclosed in U.S. Pat. No. 3,689,388. As described in detail therein, the electrolyte used consists of 86 to 93 % by weight of H.sub.2 SO.sub.4, 1.5 to 4.0% by weight of HF and 5.5 to 10.0% by weight of H.sub.2 O at a temperature of between 15.degree. and 50.degree. C, in a constant voltage of between 9 and 15 V applied to obtain damped current oscillations.
After repeating the above described steps a number of times, an excellent polishing effect is observed at the niobium surface. The oxide layer which builds up causes the oscillations to decay. Deviations of about .+-. 0.1 V from the adjusted voltage are permissible in this method. Once the voltage is switched off, the oxide layer dissolves and current oscillations again become possible when the voltage is once again swiched on. As is described in the above referenced patent, the voltage must be switched off no later than at the point of complete decay, since otherwise, the niobium surface will be etched and new surface roughness generated. The repetition of the steps results in a final product having mirror-like surfaces which can be obtained in a relatively short time. In addition, through repetition of the steps, relatively thick layers of the surface can be removed without troublesome etching. The voltage can be switched off before current oscillations have completely decayed. However, in order to get maximum benefit from the polishing action during current oscillations, the voltage should not be switched off before the maximum amplitude of the current oscillations has passed. The optimum voltage will vary, depending on the composition and temperature of the electrolyte and can be determined through experiment by increasing the voltage until the desired oscillations occur. In the above referenced patent, it has been found particularly advantageous to use an electrolyte consisting of 89.0 to 90.5% by weight H.sub.2 SO.sub.4, 2.2. to 3.0 % by weight HF and the rest H.sub.2 O at a temperature of 20.degree. to 35.degree. C and to use a constant voltage of between 11 and 13 V. With these conditions, particularly fast current oscillations occur, resulting in particularly good polishing effects.
The method of this patent is quite well suited for the preparation of mirror-smooth niobium surfaces of high surface quality and for removing entire surface layers while at the same time obtaining a polishing effect. Such surfaces are required, for example, in superconducting cavity resonators made of niobium, in which the superconductivity of niobium is used. Mirror-smooth surfaces are a great advantage in these types of devices in order to avoid high frequency or a-c losses in the superconducting niobium parts. This is also true, in particular, for superconducting niobium separators or particle accelerators and niobium conductors used in superconducting a-c cables.
Although smaller niobium parts of simple geometric shape can be electrolytically polished using the method described in U.S. Pat. No. 3,689,388 with ease, problems arise when attempting to polish the inside surface of hollow niobium bodies, in particular due to the fact that the development of gases during the electrolytic action and which rise from the cathode in the electrolyte can have a disturbing effect on the process. In particular, with hollow niobium bodies of a complicated geometrical structure, gas pockets can be formed resulting in portions of the inside surface of the niobium body not being wetted by the electrolyte and thus, not polished. In addition, the gas bubbles which flow directly along the inside surfaces of the hollow body and come into contact therewith, have a disturbing effect and can result in the supression or reduction of the oscillations required for good polishing.
A method for overcoming these difficulties has been disclosed in U.S. Pat. No. 289,543 filed on Sept. 15, 1972 and assigned to the same assignee as the present invention. In the method disclosed therein, the hollow niobium body is only partially immersed in the electrolyte and is rotatably arranged about an axis of rotation extending through the opening such that, for any position of the body, a coherent empty space in communication with the outside environment exists. As a result between the surface level of the electrolyte and all parts of the inside surface of the hollow body which are located above the electrolyte level, direct communication with the outside exists.
The hollow body is rotated so that the individual parts of the inside surface are successively immersed in the electrolyte in such that no part remains continuously therein. The cathode is introduced in the opening in the hollow niobium body and arranged in the electrolyte, relative to the hollow body, such that region of the electrolyte in which gases are formed upon passage of current permits the gases to rise to the surface of the electrolyte with the area thereabove free of parts of the inside surface of the hollow niobium body so that the gas may escape. In this method, the hollow niobium body is kept at rest during the dissolution of the oxide layer and after dissolution is rotated. The body is then stopped and a constant voltage applied again with the body at rest. These steps of applying voltage to obtain oscillation while at rest, then dissolving while still at rest, followed by a rotation, result in polishing of the entire surface without the problems associated with heavy gas development. The gas rises without touching any of the parts being polished and escapes from the empty space above the electrolyte level through the opening.
Although the method disclosed in this application makes an attempt at avoiding the formation of steps during the process, which steps result from the manner of rotation and the fact that the body remains at rest during the application of voltage and dissolution of the oxide layer, the formation of steps cannot be completely prevented at the point where the electrolyte level contacts the inside surface.
Thus, it can be seen that there is a need for an improved method of polishing which has the advantages of these previous methods, but avoids the formation of such steps.